Buffer circuit with differential structure for measurement of capacitive charges

ABSTRACT

In a buffer with an operational amplifier having two inputs and two outputs and two feedback capacitances are inserted two other capacitances which in the measurement stage are switched in parallel to the feedback ones with opposite sign in such a manner as to cancel out the effects on the output voltage signal.

BACKGROUND OF THE INVENTION

The present invention relates to a buffer circuit with differential structure for the measurement of capacitive charges.

In many electronic systems it is required to convert a capacitive charge into a corresponding voltage signal through a low-impedance circuit, usually called "buffer".

"Single-ended", i.e. nondifferential, circuits are known which perform said function but they tend to be affected by disturbances in the power supply and consequently to give quite imprecise and unsteady output signals.

Also known are reversed differential circuits which however require zeroing or resetting between one operation and the next. This complicates both the structure and the operation of said circuits.

SUMMARY OF THE INVENTION

The object of the present invention is to achieve a buffer circuit with differential structure which in a structurally and functionally simple manner would allow conversion of a capacitive charge into a steady, precise signal free of every kind of disturbance.

In accordance with the invention said object is achieved by means of a buffer circuit comprising an operational amplifier with two inputs, two outputs and a pair of feedback capacitances between said outputs and the corresponding inputs characterized in that it includes another pair of capacitances the same as said feedback capacitances and commutating devices for connecting alternately each of said other capacitances between a respective amplifier output and a polarization potential or between said amplifier output and the input corresponding to the other output.

BRIEF DESCRIPTION OF THE DRAWING

An example of a practical embodiment of the buffer circuit according to the invention is illustrated for greater clarity in the only FIGURE given in the annexed drawing which shows the circuit diagram thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the drawing, B indicates a buffer circuit including an operational amplifier A equipped with two reversed inputs I₁ and I₂ and two outputs U₁ and U₂ as well as feedback capacitances (the same) C₁ and C₂ connecting the outputs U₁ and U₂ with their repective inputs I₁ and I₂.

The output U₁ may be connected to an input polarization potential V_(P2), e.g. to earth, or alternatively to the input I₂ by means of a capacitance C₄ as a function of the state of a commutator S₂ while the output U₂ may be alternatively connected to an input polarization potential V_(P1) (the same as V_(P2)) or to the input I₁ by means of a capacitance C₃ (the same as C₂) as a function of the state of a commutator S₁ synchronized with S₂ or integral with same.

To employ a buffer circuit for the measurement of a capacitive charge such as the one stored in a capacitor C₅ and identical capacitor C₆ after the passage of a current between an input with potential V_(i) and respectively -V_(i) and a terminal with potential V_(B1) =V_(B2) =V_(P1) =V_(P2), two more pairs of switches S₃, S₄ and S₅, S₆ are provided and synchronized with S₁, S₂ or are integral with it.

In a first stage of operation all the switches S₁ -S₆ are in the position illustrated in a continuous row in the drawing and consequently the capacitors C₅ and C₆ are charged at voltage V_(i) and -V_(i) respectively while the capacitors C₁ and C₂ together with C₃ and C₄ hold steady the output voltage V_(u) and -V_(u) previously formed at the terminals U₁ and U₂.

In a second stage of operation all the switches S₁ -S₆ are moved to the position illustrated in dot and dash lines so that the capacitors C₅ and C₆, which are now charged at potentials V_(i) and -V_(i), are connected to the buffer circuit B and the capacitors C₄ and C₃ inject an equal charge of opposite sign to the one accumulated into capacitors C₂ and C₁. The result is that voltage signals V_(u), ×V_(u) are stabilized at the outputs U₁ and U₂, which reflect the charge of the capacitors C₅, C₆, i.e. they are the same as V_(i), -V_(i).

The precision of the output signals is also ensured by the capacitors C₃ and C₄ which, by injecting an equal charge of opposite sign to the one accumulated in the capacitors C₁ and C₂, cancels out their effects and in particular the phenomenon of division of the charge compared with C₅ and C₆ due to the presence of C₁ and C₂ and to the voltage memorized by them previously, which otherwise would tend to give an output signal not corresponding to the charge of the capacitor being measured. In this case the capacitor C₅, C₆ is connected to the operational amplifier A without any other comparison capacitance so that the output signal is steady, precise and unaffected by the previous operation condition.

It should also be noted that cancellation of the capacitances C₁ and C₂ by C₃ and C₄ allows in the second aforementioned stage connection to the positive input I₁ of one or more capacitors C₇ and to the negative input I₂ one or more capacitors C₈ connected to their respective generators V_(x) and -V_(x) each time it is desired to amplify these signals by a certain factor K=-C₇ /C₁ (C₇ =C₈).

This circuit therefore operates simultaneously as a buffer for certain signals and as an amplifier for other signals regardless of their number. 

We claim:
 1. Buffer circuit with differential structure for measurement of capacitive charges comprising an operational amplifier with two inputs, two outputs and a pair of feedback capacitances between said outputs and the corresponding inputs characterized in that it comprises another pair of capacitances the same as said feedback capacitances and commutating devices for connecting alternatively each of said other capacitances between a respective output of the amplifier and a polarization potential or between said amplifier output and the input corresponding to the other output.
 2. Buffer circuit according to claim 1 characterized in that it comprises other commutating devices synchronized with the abovementioned ones for connection of a capacitance being measured in parallel with said feedback capacitance simultaneously with connection of said other capacitances between said amplifier output and the input corresponding to the other output.
 3. Buffer circuit according to claim 1 characterized in that said polarization potential is equal to the amplifier input polarization potential. 